Florastor is the probiotic to own before you need it, because the moment you need it tends to arrive without warning: a doctor hands you a course of antibiotics. Here's the catch almost everyone misses — the standard bacterial probiotics (Lactobacillus, Bifidobacterium) get killed by the same drug they're meant to coexist with, so taking your usual probiotic alongside antibiotics largely wastes it. Florastor sidesteps the problem entirely by not being a bacterium at all. It's Saccharomyces boulardii CNCM I-745, a probiotic yeast, and antibacterial antibiotics have no mechanism to kill a yeast. That single biological fact is the whole product: it's why S. boulardii has the strongest evidence for antibiotic-associated diarrhea, why you can take it during the course rather than after, and why it survives stomach acid and ships shelf-stable. The trade-off is that Florastor is a specialist, not a daily generalist — a yeast won't colonize your gut with the bacteria that do everyday microbiome work. Here's the full breakdown of when it's exactly right, and when it isn't.
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Read the complete Probiotics guide →Exact strain matched to a specific, well-evidenced indication. Florastor delivers Saccharomyces boulardii CNCM I-745, the studied form behind the antibiotic-associated-diarrhea data — McFarland 2010 (PMID 20458757) found 8 of 10 adult RCTs showed significant efficacy (pooled RR 0.47 for AAD prevention). Named to the code, matched to the problem. Below a perfect score only because the indication is narrow, not the match.
The standout mechanism on the entire list. As a yeast, S. boulardii survives stomach acid well AND is naturally resistant to antibacterial antibiotics — so unlike every bacterial pick, it isn't killed by the drug it's taken alongside. The substance hub flags exactly this as S. boulardii's advantage for the AAD use-case. Best-in-class survivability for its specific job.
Complete for one job, thin as a daily tool. A single yeast at the studied 250 mg dose with no prebiotic and no supporting bacterial strains — precisely right for antibiotic-associated and acute diarrhea, and precisely wrong if you want broad ecosystem coverage. It isn't a synbiotic like Seed (#1) or a multi-strain blend; it's a targeted single-organism specialist and doesn't pretend otherwise.
Strong on strain identity — named down to the CNCM I-745 code, so you know you're getting the studied organism that the AAD evidence is built on, not a generic 'S. boulardii.' Florastor is a long-established, clinically-positioned brand for this use. Short of top marks because there's no prominent public COA or NSF certification surfaced for the consumer line.
Roughly $0.67 per 2-capsule daily dose (~$36/month), mid-pack — pricier per day than budget bacterial blends, but you're buying a yeast specialist, not a commodity. The 54-count bottle is sensible for a course-length or keep-on-hand use rather than indefinite daily dosing. Fair value for the specific antibiotic-recovery job; poor value if misused as a daily generalist.
“A yeast — antibiotics can't kill it, so you can take it during a course.”
Mechanistically correct and the central differentiator. Saccharomyces boulardii is a yeast, and antibacterial antibiotics have no mechanism to kill yeast — the substance hub explicitly notes this as S. boulardii's advantage for the antibiotic-associated-diarrhea use-case. Unlike every bacterial probiotic here, it can be co-administered with the antibiotic. Real and the reason it's the post-antibiotic pick.
“Evidence-backed for antibiotic-associated diarrhea.”
Well substantiated for this specific indication. McFarland 2010 (PMID 20458757) found 8 of 10 adult S. boulardii RCTs showed significant efficacy (pooled RR 0.47 for AAD prevention), and McFarland 2006 (PMID 16635227) found probiotics as a class significantly cut antibiotic-associated diarrhea and helped with C. difficile disease. Florastor uses the studied CNCM I-745 strain. Strong indication-specific support — for AAD, not general wellness.
“Uses the studied strain Saccharomyces boulardii CNCM I-745.”
Florastor names the strain down to the CNCM I-745 code — the specific form behind much of the S. boulardii antibiotic-associated-diarrhea evidence. Strain-level naming is exactly the transparency the category needs, and it's how you confirm you're buying the studied organism rather than a generic yeast. Verifiable from the label.
“Shelf-stable — no refrigeration required.”
Consistent with the substance hub, which lists S. boulardii among the engineered shelf-stable options that survive without cold storage. A real convenience advantage for a keep-on-hand bottle you reach for on short notice when antibiotics are prescribed. Confirm the storage line on your specific lot, but room-temperature stability is genuine here.
“A daily probiotic for everyday digestive health.”
Misleading if read as a general-gut-health claim. Florastor's evidence is for antibiotic-associated and acute diarrhea, not broad daily maintenance — and as a yeast it won't colonize the gut with the Lactobacillus/Bifidobacterium bacteria that do everyday microbiome work. It's an excellent specialist for its indication, but 'daily' shouldn't be read as 'everyone's everyday probiotic.'
Florastor's whole value rests on one biological fact: Saccharomyces boulardii is a yeast, and antibacterial antibiotics can't kill yeast. That's why it's the rare probiotic you can take during an antibiotic course instead of waiting until after, and why the substance hub singles out S. boulardii's antibiotic-resistance as its advantage for the antibiotic-associated-diarrhea use-case. Every bacterial probiotic on our list — Lactobacillus, Bifidobacterium — gets wiped out by the same drug it's meant to coexist with. Florastor doesn't, and that single mechanism is what earns its place on the list.
The defining feature of the antibiotic-associated-diarrhea use-case is timing: you find out you need a probiotic the moment a doctor writes a prescription, often with no time to research. Florastor being shelf-stable (no fridge, per the substance hub's note on engineered S. boulardii products) makes it the bottle to keep in the medicine cabinet or travel bag for exactly that moment. Start it when you start the antibiotic, space the doses a couple of hours apart, and continue for several days to a week after the course — that's the protective protocol the hub describes.
The honest limitation, and the reason this lands at 'consider' rather than 'buy' for most readers: a yeast is the wrong tool for general daily gut health. Florastor won't colonize your gut with the Lactobacillus/Bifidobacterium species that do most of the everyday microbiome work, and its evidence is specifically for antibiotic-associated and acute diarrhea — not broad maintenance. If your goal is general gut health and bloating, a broad bacterial synbiotic like Seed (#1) is the right pick. Use Florastor for the antibiotic episode, and run a bacterial probiotic as your daily.
Where many probiotic claims rest on class-level or thin single-product data, Florastor's core claim is well-supported. McFarland 2010 (PMID 20458757) reviewed 10 adult S. boulardii RCTs and found 8 of 10 showed significant efficacy, with a pooled relative risk of 0.47 for antibiotic-associated diarrhea — roughly a halving of risk. The broader McFarland 2006 meta-analysis (PMID 16635227) reached the same conclusion for probiotics as a class, including a role against C. difficile disease. For its one indication, Florastor is on solid ground.
Two things separate Florastor from a generic 'S. boulardii' bottle. First, it names the strain down to the CNCM I-745 code — the specific form the evidence is built on, exactly the transparency the category needs. Second, as a yeast it survives stomach acid well without needing the enteric or delayed-release engineering bacterial probiotics rely on. Neither is marketing fluff: strain identity is what ties the product to the trials, and acid-survival is what gets the organism to where it acts. For the antibiotic-recovery job, both matter.
Florastor is the answer to one situation that catches everyone out: you've just been prescribed antibiotics. Standard bacterial probiotics get killed by the same drugs, but Florastor is Saccharomyces boulardii — a yeast — so it survives, which is exactly why it's the evidence-backed option for antibiotic-associated diarrhea (McFarland 2010, PMID 20458757) and why you can take it during the course rather than waiting until after. It's shelf-stable, acid-resistant, and named to the studied CNCM I-745 strain. For the antibiotic-recovery job specifically, it's the clearest pick on the shelf, and the bottle worth owning before you need it. The reason it lands at 'consider' rather than 'buy' is that it's a specialist, and most people most of the time aren't on antibiotics. Outside of antibiotic recovery or acute/traveler's diarrhea, a yeast won't colonize your gut with the Lactobacillus/Bifidobacterium species that do everyday microbiome work — so for general gut health and bloating, a broad bacterial synbiotic like Seed (#1) is the right call, and for IBS-type pain it's the studied bacterial strain in Align (#6). The smart play is to keep Florastor on hand for the next prescription, run a bacterial probiotic as your daily, and reach for the yeast precisely when — and only when — antibiotics enter the picture.
Check Florastor · Saccharomyces boulardii CNCM I-745 (yeast), 250 mg, 54ct, antibiotic-resistant on AmazonThe everyday bacterial probiotic Florastor isn't — a broad 24-strain synbiotic, survivability-engineered with a built-in prebiotic. The right daily pick once the antibiotic episode is behind you.
See it on the list →The other strain with antibiotic-associated-diarrhea evidence — LGG (Szajewska 2015), a bacterium, so best continued after the course rather than during it. A sensible bacterial companion around antibiotics.
See it on the list →The targeted pick for a different job — B. infantis 35624, the studied strain for IBS-type pain and bloating (Whorwell 2006). Buy the strain, not the yeast, if IBS is your problem.
See it on the list →Reviewed 10 adult RCTs of Saccharomyces boulardii for antibiotic-associated diarrhea: 8 of 10 showed significant efficacy, pooled relative risk 0.47 for AAD prevention. Establishes S. boulardii — a yeast, so antibiotic-resistant — as a leading strain for the antibiotic-recovery use-case, and the direct evidence behind Florastor's core claim.
Pooled the RCTs and found probiotics significantly reduced the risk of antibiotic-associated diarrhea and helped in the treatment/prevention of Clostridium difficile disease. The cornerstone meta-analysis behind probiotics' strongest indication — the context in which an antibiotic-resistant yeast like Florastor's S. boulardii is the targeted choice during a course.
Meta-analysis of 12 RCTs (1,499 participants): Lactobacillus rhamnosus GG reduced the risk of antibiotic-associated diarrhea from 22.4% to 12.3% versus control. The other best-studied AAD option — a bacterium, so unlike Florastor's yeast it's best continued after the course rather than co-administered during it.
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